Method and apparatus for preboiler cleanup of a once-through steam generator



July 13, 1965 H. A. GRABOWSKI METHOD AND APPARATU S FOR PREBOILER CLEANUP 0 A ONCE-THROUGH STEAM GENERATOR Filed March 25, 1963 FURNACE INVENTOR HILARY A. GRABOW SKI WI. UAW

United States Patent M METHDD AND APPARATUS FOR PREBOILER CLEANUP OF A ONCE-THROUGH STEAM GENERATOR Hilary A. Grabowski, West Simsbury, Conn., assignor to Combustion Engineering, Inc, Windsor, Conn., a corporation of Delaware Filed Mar. 25, 1963, Ser. No. 267,515 4 Claims. (Cl. 122-379) This invention relates to a forced flow once-through steam generator, and in particular to a method of operation for cleaning the fluid circuit associated with such a forced flow once-through steam generator.

In a once-through steam generator it is important to maintain the fluid flowing through the fluid circuit at a relatively high degree of purity in order to prevent the formation of deposits on the tubes within the heat exchange members, and also on the turbine blades. Deposits in boiler tubes could lead to overheating and failtire of the tube metal; deposits on the turbine blades results in high pressure drops across the steam turbine stages, thus reducing the output capacity of the turbine. For this reason, the fluid supplied to the steam generator is continuously filtered, demineralized, and de-aerated during operation; and on occasion, generally just before eachstartup of the unit, the fluid circuit is flushed to remove any deposits of salts or other corrosive materials which may have been formed or become trapped in any of the heat exchanger tubes, and interconnecting piping. It is an object of this invention to provide novel means and a method of operation thereof for quickly and efficiently cleaning the fluid circuit associated with a forced flow once-through steam generator. ()ther objects and advantages of the invention will become apparent from the following specification and accompanying drawing wherein the figure shows a diagrammatic representation of a forced fiow once-through steam generator system embodying my invention.

Looking now to the drawing, broken line represents a furnace and its associated gas pass, through which hot combustion gases flow. Fuel and air are supplied to the furnace by Way of burner 11, which fuel is burned in the furnace 10 to thereby generate the hot combustion gases.

Contained within the furnace and its associated rear gas pass is a fluid circuit comprised of economizer 12, furnace wall steam generating tubes 14, superheater 18, and reheater 28. Numeral 16 designates a boiler throttle valve arrangement which is used during startup and other critical times.

Fluid enters the circuit contained within the furnace 'and its associated gas pass flowing first through the This heated water then enters the tubes 14 lining 3,194,216 Patented July 13, 1965 ICC During startup of the unit steam must be slowly admitted to the turbine in order to prevent thermal shock thereto. Thus it is necessary to bypass some of the steam produced in the steam generator during the startup cycle. For this reason the line leading to the high pressure turbine 20 contains a shutoff valve 22, and a bypass line 24 is provided containing shutoff valve 26 which extends from a point upstream of valve 22 to the condenser 32.

The feedwater. system is made up of feedwater line 60 the inlet of which is supplied with water from the condenser 32 and the outlet of which is connected to the economizer 12. Positioned within this feedwater line 6i) is a condensate pump 34, filter and demineralizer 36,

driven by steam generated in the auxiliary boiler '68; The

steam discharged from turbine 70 is used for heating the feedwater flowing through de-aerator 46. Line 71 completes the circuit for the auxiliary boiler 65.

As shown, the low pressure feedwater heater is made up of two heaters 38 and 40 which are in parallel flow relationship with one another. The branched lines leading to the heaters 33 and 40 contain shutoff valves 42 and 44 respectively. Similarly, the high pressure feedwater heater is made up of two feedwater heaters and 52 which are also in parallel. The branched lines leading to these two heaters contain shutoff valves54 and 56 respectively. The purpose of these parallel heaters and the shutoff valves in such parallel lines will be described more fully at a later time. i A bypass line 64 extends from the outlet of feedwater line 60 to the hotwell 33 and contains a shutoff valve 66. Bypass line 64 is connected to the feedwater line outlet upstream of the feedwater shutoif valve 62.

The operation of the system will now be described. When it is desired to clean or flush out the fluid circuit associated with the once-through steam generator, feedwater shutoff valve 62 is closed, and the valve 66 in the bypass line 64 is opened. Feedwater pump 48 is started up, thus forcing water through the feedwater system and the bypass line 64. The fluid is allowed to continue to circulate through this circuit until a predetermined degree of purity of the water is sensed by any wellknown chemical analyzer, 58, which is shown positioned within the feedwater line 60 downstream of the high pressure feedwater heaters. During this preboiler cleanup operation, any deposits of foreign material which may have formed, or had become trapped within the feedwater heater sections will be flushed out, and removed from the water as it passes through the filter and demineralizer 36. Device 36 removesboth the soluble and insoluble impurities from the water.

The amount of power required to drive the feedwater pump 48 during full load operation is quite large, and it is not economical to drive the pump at this high rating during the cleanup operation. The turbine 70 is thus operated during the startup cycle such that it will drive the feedwater pump 48 at only a minor percentage of creased flow therethrough during the cleanup cycle.

the rate at which it is driven during full load operation, for example 15% of full load operation.

It is well recognized that an increase in temperature and in flow rate results in a more efiicient and quick cleanup cycle. Thus the temperature of the Water being circulated is heated in the de-aerator to a temperature of approximately 230 F. during the preboiler cleanup cycle. The high and low pressure feedwater heaters have been constructed of parallel heaters in order to enable an in- Thus if shutoif valve 42 is closed during a portion of the cleanup cycle, and all of the fluid is allowed to flow through heater 4%, the flow rate therethrough will be twice as large as it would have been if the flow had been allowed to pass through both of the parallel heaters. The same is true of the high pressure heaters 5t and 52.. Thus during the first part of a cleanup cycle valves 42 and 54 would be closed, forcing the entire flow to pass through the low pressure heater 4t) and the high pressure heater 52. When analyzer 533 senses a predetermined degree of cleanliness of the water, valves 42 and 54 can be opened, and valves 44 and 56 closed. The cycle is then repeated until the other two heaters 38 and 50 are cleaned out and analyzer 58 again senses a predetermined degree of purity. The water should be cleaned to a high degree of purity, for example 20 parts per billion of foreign material. At this time shutoff valve 66 in the bypass line 6 5 is closed, and the feedwater valve 62 is opened. The heat exchange members positioned Within the furnace and its associated gas pass can thereafter be cleaned.

Obviously, the feedwater heaters can contain more than two sections in parallel. For example, if three were in parallel, and two of them were shut-01f, the flow rate through the remaining one would be three times greater than would be the case if all of them were open to flow.

By providing the steam generator with bypass line 64, it is possible to first clean the feedwater system removing the impurities from the elements within this system, and thereafter cleaning the portion of the fluid circuit contained within the furnace. Without this bypass line all of the impurities flushed out of the feedwater system would be carried on into the boiler circuit contained Within the furance. Some of these impurities thus flushed into the furnace circuit could settle out or become entrapped within various parts of this circuit. This is especially true due to the fact that the flow rate established by feedwater pump 48 during the boiler cleanup cycle is relatively small.

Although only one embodiment of the invention has been shown and described, it is to beunderstood that the invention is not to be limited thereto. The invention could be utilized in other alternative arrangements. For example although a monotube once-through type unit has been illustrated, the invention would apply equally as well to a once-through steam generator including a recirculation system such as described in my co-pending application filed on even date herewith and entitled Boiler Cleanup Method for Combined Circulation Steam Generator, Serial No. 267,588.

What I claim is:

it. In a forced flow once-through steam generator having a fluid circuit, the fluid circuit including a feedwater system through which water flows, said feedwater systern including a feedwater line having an inlet and an outlet, feedwater heating means positioned within said feedwater line a feedwater pump in said feedwater line, means positioned in the feedwater line for removing impurities from the Water, a furnace including fuel burning means for creating hot combustion gases, heat exchange means positioned within the furnace for generating steam therein, said heat exchange means having an inlet and an outlet, the outlet of said feedwater line being connected to the heat exchange means inlet, a first shutoff valve positioned in said feedwater line prior to its outlet, a bypass line having an inlet and an outlet, a second shutoff valve positioned in the bypass line, the bypass line inlet being connected to the feedwater line upstream of the first shutoff valve and the bypass line outlet being connected to the feedwater line inlet, whereby when it is desired to clean the fluid circuit the first shutoff valve can be closed and the second shutoff valve opened to permit fluid to be circulated through the feedwater system and said bypassline, and when the water therein reaches a predetermined purity, the first shutoff valve can be opened and the second shutoff valve closed to permit cleaning of the heat exchange means.

2. In a force flow once-through steam generator having a fluid circuit, the fluid circuit including a feedwater sys tern through which water flows, said feedwater system including a feedwater line having an inlet and an outlet, first and second feedwater heating means positioned within said feedwater line, said first and second feedwater heating means being in parallel flow relationship with one another, said feedwater line containing branched lines leading to and from both said first and second feedwater heating means, a first shutoff valve positioned in one of the branched lines, a second shutoff valve positioned in the other branched line, a pump in said feedwater line, means positioned in the feedwater line for removing impurities from the water, a furnace including fuel burning means for creating hot combustion gases, heat exchange means positioned within the furnace for generating steam therein, said heat exchange means having an inlet and outlet, the outlet of said feedwater line being connected to the heat exchange means inlet, a third shutoff valve positioned in said feedwater line prior to its outlet, a bypass line having an inlet and an outlet, a fourth shutoff valve positioned in the bypass line, the bypass line inlet being connected to the feedwater line upstream of the third shutoff valve, and the bypass line outlet being connected to the feedwater line inlet.

3; A method of cleaning the fluid circuit of a forced flow once-through steam generator, the steam generator comprising a furnace, a fluid circuit including a feed- Water system having an inlet and an outlet, a steam generating circuit positioned in the.furnace,.which steam generating circuit is connected to the outlet of the feedwater system, a bypass extending from the feedwater system outlet to the feedwater system inlet, the feedwater system including a pump, feedwater heating means, and means for removing impurities from the fluid, which method comprises the steps of starting the pump to force fluid through the feedwater system, passing all of the pumped fluid through the bypass until the fluid reaches a predetermined degree of purity to thus clean 'out the feedwater system, and thereafter passing all of the pumped fluid into the steam generating circuit to thus clean the steam generating circuit. a

4. A method of cleaning the fluid circuit of a forced flow once-through steam generator, the steam generator comprising a furnace, a fluid circuit including a feedwater system having an inlet and an outlet, a steam generating circuit positioned in the furnace, which steam generating circuit is connected to the outlet of the feedwater system, a bypass extending from the feedwater system outlet to the feedwater system inlet, the feedwater system including a pump means, at least two feedwater heaters in parallel, and means for removing impurities from the Water, Which method comprises the steps of operating the pump means to force fluid through the feedwater system at a rate which is a predetermined minor percentage of the rate at which the fluid flows when the steam generator is operating at full load capacity, passing all of the fluid through one of the feedwater heaters and then through the bypass until the fluid reaches a predetermined degree of purity, thereafter successively passing all of the fluid through each of the remaining feedwater heaters and the bypass until the fluid reaches a predetermined degree of purity, and thereafter passing 5 6 all of the pumped fluid into the steam generating circuit 2,907,305 10/59 Profos 122379 to thus clean the steam generating circuit. 3,009,325 11/61 Pirsh 122-1 3,010,853 11/61 Elliott 13413 References Cited by the Examiner 3,085,915 4/63 Heitmann et a1.

UNITED STATES PATENTS 5 FOREIGN PATENTS 1,064,855 6/13 Parry. 617,110 3/61 Canada. 1,791,923 2/31 Eule 122-379 X 386,042 12/23 Germany. 1,892,093 12/32 Battistella. 2 55 12 51 Webb 134 22 10 PERCY L. PATRICK, Przmary Examiner. 2,858,808 11/ 58 ROWand 122379 FREDERICK KETTERER, Examiner. 

1. IN A FORCED FLOW ONCE-THROUGH STEAM GENERATOR HAVING A FLUID CIRCUIT, THE FLUID CIRCUIT INCLUDING A FEEDWATER SYSTEM THROUGH WHICH WATER FLOWS, SAID FEEDWATER SYSTEM INCLUDING A FEEDWATER LINE HAVING AN INLET AND AN OUTLET, FEEDWATER HEATING MEANS POSITIONED WITHIN SAID FEEDWATER LINE, A FEEDWATER PUMP IN SAID FEEDWATER LINE, MEANS POSITIONED IN THE FEEDWATER LINE FOR REMOVING IMPURITIES FROM THE WATER, A FURNACE INCLUDING FUEL BURNING MEANS FOR CRATING HOT COMBUSTION GASES, HEAT EXCHANGE MEANS POSITIONED WITHIN THE FURNANCE FOR GENERATING STEAM THEREIN, SAID HEAT EXCHANGE MEANS HAVING AN INLET AND AN OUTLET, THE OUTLET OF SAID FEEDWATER LINE BEING CONNECTED TO THE HEAT EXCHANGE MEANS INLET, A FIRST SHUTOFF VALVE POSITIONED IN AID FEEDWATER LINE PRIOR TO ITS OUTLET, A BYPASS LINE HAVING AN INLET AND AN OUTLET, A SECOND SHUTOFF VALVE POSITIONED IN THE BYPASS LINE, THE BYPASS LINE INLET BEING CONNECTED TO THE FEEDWATER LINE UPSTREAM OF THE FIRST SHUTOFF VALVE AND THE BYPASS LINE OUTLET BEING CONNECTED TO THE FEEDWATER LINE INLET, WHEREBY WHEN IT IS DESIRED TO CLEAN THE FLUID CIRCUIT THE FIRST SHUTOFF VALVE CAN BE CLOSED AND THE SECOND SHUTOFF VALVE OPENED TO PERMIT FLUID TO BE CIRCULATED THROUGH THE FEEDWATER SYSTEM AND SAID BYPASS LINE, AND WHEN THE WATER THEREIN REACHES A PREDETERMINED PURITY, THE FIRST SHUTOFF VALVE CAN BE OPENED AND THE SECOND SHUTOFF VALVE CLOSED TO PERMIT CLEANING OF THE HEAT EXCHANGE MEANS. 